DNS Example

Here is the hierarchy for CLUE: clue.denver.co.us.
The top of the hierarchy is on the right (the dot, which is usually optional for things humans use).
When a client queries a server, the server asks each layer in turn until it finds an answer:
- . (root) ([a-m].root-servers.net)
- us. ([a-c].gtld.biz)
- (Notice there is no level for co.us.)
- denver.co.us. (ns1.westnet.net and ns.utah.edu)
- clue.denver.co.us. (dns0.techangle.com et. al.)
The server caches all the answers it gets so the process is not so painful next time.

So, how does a lookup happen? A client (your web browser, for example) sends a query to a nameserver (listed in /etc/resolve.conf–probably your ISP's nameserver):

client to ISP: "Tell me the address for clue.denver.co.us."

ISP to a.root-servers.net: "Tell me the address for clue.denver.co.us."

a.root-servers.net to ISP: "Don't know, but a, b, and c.gtld.biz know the us. zone (they are authoritative for us.)."

ISP to a.gtld.biz: "Tell me the address for clue.denver.co.us."

a.gtld.biz to ISP: "Don't know, but ns1.westnet.net and ns.utah.edu know the denver.co.us. zone." (Notice there is no co.us. level in the hierarchy.)

ISP to ns1.westnet.net: "Tell me the address for clue.denver.co.us."

ns1.westnet.net to ISP: "Don't know, but dns0.techangle.com (and others) know the clue.denver.co.us. zone."

ISP to dns0.techangle.com: "Tell me the address for clue.denver.co.us."

dns0.techangle.com to ISP: "clue.denver.co.us has address 199.239.19.17"

ISP to client: "clue.denver.co.us has address 199.239.19.17"